Cobalt-catalyzed selective dehydrocoupling polymerization of prochiral silanes and diols
Graphical abstract
A cobalt-catalyzed selective dehydrocoupling polymerization was presented, resulting in novel PSEs with pendant SiH groups. Different types of monomers are suitable substrates for the polymerization. A series of PSEs were provided with good yields and high molecular weight. Asymmetric dehydrocoupling polymerization was also realized by employing the chiral bisphosphine ligand, giving the chiral PSE with 65% ee.
Introduction
Because of possessing various properties and abundant reserves of Si and O in the earth’s crust [1], developing polymer containing a silicon-oxygen bond in the mainchain has been a hot topic. Until now, several polymeric materials containing SiO bond in the mainchain have been developed and applied, including polysiloxanes, polysilylethers, polysilylesters, etc. [2]. These polymeric materials usually exhibit similar properties, such as low Tg, good thermal stability, biocompatibility, high gas permeability, degradability, etc. [3]. Among these polymers, polydimethylsiloxane (PDMS) possesses high flexibility and thermal stability, which has been successfully used as silicon oil, elastomer, adhesives, coatings, etc. [4], [5], [6].
Due to the combination of favorable properties of polycarbosilanes and polysiloxanes, polysilylethers have gradually attracted the attention in the past few decades [2]. In the early work, the synthesis of PSEs was often realized by the polycondensation of diols and dichlorosilanes or dianilinosilanes [7]. However, equivalent base or vacuum condition is required to furnish high-Mn PSEs, which has negative influence on the atom-economy and further application of polymerization [8]. Afterwards, Nishikubo’s group synthesized a new kind of polysilylethers with reactive pendant chloromethyl groups via TPBC catalyzed polyaddition of bis(epoxide)s and volatile dichlorosilanes [9]. In order to replace the unstable chlorosilanes, silanes were gradually applied to the synthesis of PSEs via dehydrocoupling [10] and hydrosilylation polymerization [11].
As an efficient and high atom-economic way to synthesize PSEs, dehydrocoupling polymerization has been catalyzed by the catalysts derived from precious transition metals such as palladium, platinum, rhodium, and ruthenium [12]. Various silanes and diols were used for providing PSEs with different thermal stability and degradability. In despite of these advance, drawbacks of these catalytic systems based on noble metals, such as high cost, low abundance, high catalyst loading and biological compatibility become increasingly the key problem of its development [13]. Thus, use of catalysts based on first-row transition metals like iron (Fe), manganese (Mn) and cobalt (Co) is much more sustainable for addressing the disadvantages of precious metals. Recently, the catalytic systems derived from manganese and iron have been reported [14]. However, Co-catalyzed dehydrocoupling polymerization of silanes and diols has not been developed. Moreover, another problem of the development of PSEs is that most of the PSEs reported before have similar structure and framework, which is unfavorable for adjusting its properties and expanding its applications. In 2000, Rh-catalyzed selective dehydrocoupling of prochiral silanes and diols was developed by the group of Kawakami, giving novel PSEs with pendant SiH groups and up to 39.8% average ee of silicon atoms [15]. The reactive SiH groups of these PSEs may react with water or divinyl compounds, which will have potential in the application as chiral column packing materials. Although the stereoselectivity of the polymerization was still not satisfactory, these results provided a new developing trend of PSEs. In the context of our continuous interest in transition-metal-catalyzed dehydrocoupling polymerization [16], herein we reported cobalt-catalyzed selective dehydrocoupling polymerization of prochiral silanes and diols, furnishing a series of high-Mn PSEs containing pendant Si-H groups. Furthermore, an asymmetric version of this transformation was also studied; moderate 65% of enantioselectivity was obtained. (Scheme 1).
Section snippets
Results and discussion
Original experiments employed silane 1a and diol 2a at 60 °C in the presence of Co(acac)2/dppb as a catalyst (Table 1). Initially, the reaction was conducted in THF, giving a polymer with moderate Mn and high yield (entry 1). When the ligand was removed, an oligomer was obtained, which indicated phosphine ligand play a vital role in the polymerization (entry 2). Then, the solvent effect was screened. It was found that high yields were also achieved in MeCN, tBuOMe and nhexane (entries 3,5,7).
Conclusions
In summary, we have developed Co-catalyzed selective dehydrocoupling of prochiral silanes and diols with a catalyst generated from Co(acac)2 and dpppe. Different types (AB type or AA and BB type) of monomers are suitable substrates for the polymerization. A series of novel PSEs with pendant Si-H groups were provided with good yields (up to 89% yield) and high molecular weight (up to 32.3 kg/mol). Moreover, the Co-catalyzed one-pot two-step process was realized. An asymmetric version of
Reagents and instrumentation
All reactions were carried out under an atmosphere of nitrogen using the standard Schlenk techniques, unless otherwise noted. Diols 2a-f were recrystalized from ethyl acetate. Other commercially available reagents were used without further purification. Solvents were treated prior to use according to the standard methods. 1H NMR and 13C NMR spectra were recorded at room temperature in CDCl3 on 400 MHz instrument with tetramethylsilane (TMS) as internal standard. Flash column chromatography was
CRediT authorship contribution statement
Xiao-Yong Zhai: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing - original draft. Xiao-Qing Wang: Investigation. Yong-Gui Zhou: Supervision, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
Financial support from National Natural Science Foundation of China (21690074), Dalian Institute of Chemical Physics (UN201701) and Chinese Academy of Sciences (XDB17020300) is acknowledged.
References (22)
- et al.
Angew. Chem. Int. Ed.
(2016)et al.J. Am. Chem. Soc.
(2004) - W.M. Haynes, Abundance of Elements in the Earth’s Crust and in the Sea, CRC Handbook of Chemistry and Physics, 96th...
- et al.
Des. Monomers Polym.
(2000) - et al.
Silicones
Ullmann’s Encyclopedia of Industrial Chemistry
(2005) - et al.
In Silicon Based Polymer Science: A Comprehensive Resource
ACS Press
(1990) Rubber Chem. Technol.
(1965)- et al.
Rubber Chem. Technol.
(1990) - et al.
Macromolecules
(1968)et al.J. Polym. Sci., Part A: Polym. Chem.
(1967)et al.Macromolecules
(2013)et al.J. Polym. Sci., Part A: Polym. Chem.
(1999)et al.J. Polym. Sci., Part A: Polym. Chem.
(1990)J. Macromol. Sci., Chem.
(1991) - et al.
J. Polym. Sci., Part A: Polym. Chem.
(1994) - et al.
Macromolecules
(1995)et al.Polym. J.
(1993)et al.J. Polym. Sci. Part A: Polym. Chem.
(2000)et al.Macromolecules
(1996)
ACS Symp. Ser.
Cited by (12)
Catalytic silylation of O–nucleophiles via Si–H or Si–C bond cleavage: A route to silyl ethers, silanols and siloxanes
2022, Coordination Chemistry ReviewsCitation Excerpt :Lastly, the latter undergoes reductive elimination to provide dihydrogen and regenerated catalyst (Fig. 51). Later, the same authors expanded this method by using anionic iridium species [173], or less expensive cobalt-[174] and copper-based systems [175]. In subsequent contributions, the researchers utilized the group 10 metals.
Synthesis of poly(silyl ether)s via copper-catalyzed dehydrocoupling polymerization
2022, Chinese Chemical LettersThe recent advances in cobalt-catalyzed C(sp<sup>3</sup>)-H functionalization reactions
2022, Organic and Biomolecular Chemistry